Structure Of The Milky Way
The problem we face trying to deduce the structure of the Milky Way from our location within it is analogous to the problem faced by the Amazon Indians trying to map the rain forest while confined to its boundaries: it is simply too vast. To map the rainforest today we can simply fly over it in a plane; we cannot yet fly out of the galaxy in a spaceship to map its structure. We must find other methods.
Clues to the structure of our galaxy can be found by examining other galaxies similar to our own. The Milky Way has a disk structure with two spiral arms winding out from the center in the plane of the disk. The center, or nucleus, contains a small bulge. Surrounding this disk shape is a spherical halo composed of globular clusters similar to those used by Shapley to deduce our location within the Milky Way. "Similar Galaxies" have some overall structure.
We map the spiral arm structure of the Milky Way using ordinary optical light and looking for objects commonly found in spiral arms of other galaxies. To map the largest region possible we use bright objects. These objects, known as spiral arm tracers, include O and B spectral class stars, O and B associations, HII regions, and Cepheid variable stars.
Astronomers put stars into different spectral classes. The O and B spectral classes are the two with the brightest and most massive stars. O and B associations are loose clumps of roughly a few dozen or so O and B stars. HII regions are clouds of ionized hydrogen surrounding very recently formed O or B stars. Cepheid variable stars vary in brightness in a particular way. The first member of this class was discovered in the constellation Cepheus, hence the name. These stars are one of the fundamental yardsticks used by astronomers to measure distances in the universe, so they can be used to find the distance to the spiral arm containing them. These spiral arm tracers allow us to map the spiral arm structure of the Milky Way. We can only map a small part, however, as interstellar dust blocks the optical light from the more distant parts of the galaxy.
Astronomers use radio waves to map the far reaches of the Milky Way because the interstellar dust does not block radio waves as much as optical light. Spiral arms also contain interstellar gas composed mostly of hydrogen atoms. This interstellar gas is so thin (on average slightly less than one hydrogen atom per cubic centimeter of space) that it would be an excellent vacuum on Earth, but interstellar space is so vast that the interstellar gas still adds up to a lot of hydrogen atoms. These hydrogen atoms emit radio waves with a wavelength of 8 in (21 cm) that can penetrate the interstellar dust. The 21-cm radio waves allow us to map the spiral arm structure of even the distant parts of the Milky Way.
Using these and other techniques, astronomers have deduced the structure, size, and content of the Milky Way. The Milky Way consists of a fairly flat disk about 120,000 light years in diameter and 1,000 light years thick. (A light year is the distance light travels in one year, about six trillion miles.) The edge of the galaxy has a fuzzy rather than a sharp boundary, so the size estimates depend on what one calls the edge.
The flat disk consists of a complex spiral pattern, rather than the two graceful arms found in some galaxies. In addition to the spiral arm tracers mentioned previously, the disk and spiral arms contain young stars of all spectral classes, galactic clusters composed of several hundred young stars, and interstellar clouds of gas, molecules and dust where new stars form. There is some recent evidence that the spiral arms do not begin at the center of the galaxy but at either end of a central bar structure like those found in barred spiral galaxies. The Sun and solar system are located on a spiral arm about 25,000 light years from the center.
The nucleus of the galaxy is surrounded by a nuclear bulge that is 12,000 light years in diameter and 10,000 light years thick. Surrounding this disk is a spherical halo, composed primarily of globular clusters. The halo may be as much as 300,000 light years in diameter and contains a considerable amount of unseen dark matter, perhaps as much as several times the amount of mass that we can see. The extent of the dark matter is difficult to determine because it is not clearly defined and not yet measurable. Some astronomers suggest that the dark matter portion of the halo may extend as far as half the distance to the Andromeda galaxy.
In 1997, astronomers discovered an astounding sight—a fountain of hot gas and antimatter shooting some 3,500 light years out of the nucleus of the Milky Way perpendicular to the disk. The discovery was made by the Compton Gamma Ray Observatory, which registered the massive flow of gamma rays emitted by the antimatter. Astronomers were unclear whether the antimatter jets are continuous or whether they are merely observing an antimatter cloud slowly separating from the rest of the galaxy.
The fountain appears to primarily contain positrons, rather than more massive antimatter particles or atoms. Positrons can be created by heated gas spiraling into a black hole, or by the explosions of supernovas and white dwarf stars, leading astronomers to speculate that the fountain is caused by massive star formation near the black hole at the Milky Way's center, or the explosion of young massive stars.
How many stars are there in the Milky Way? The spinning provides clues to the answer. The Milky Way spins because the individual stars in the galaxy are orbiting the center of the galaxy. Just as the Sun's gravity causes the planets to orbit the sun, the cumulative gravitational effect of the stars in the Milky Way cause the stars farther out to orbit the center of the Milky Way. The amount of gravitational force and hence the orbital properties depend on the mass.
We can therefore study the orbital motions of the outer stars in the Milky Way to find the mass of the Milky Way. The mass of the Milky Way within a diameter of 120,000 light years is about 3 hundred billion times the mass of the Sun. Because the Sun is a fairly average star the Milky Way contains roughly 2 to 3 hundred billion stars. The orbital motions of the Sun are such that it moves around the center of the galaxy at about 220 km/s and takes about 250 million years to orbit the galactic nucleus once.